Particle beam apparatus having an imaging lens which is provided with an associated phase-displacing foil

ABSTRACT

A particle beam device has a longitudinal axis and a beamgenerating portion for issuing particle beams along the axis. A holder is provided for accommodating a specimen in the path of the beams and a particle beam imaging lens is disposed beyond the specimen locality coaxial with the axis. A foil is disposed in the lens in the path of the particle beams for shifting the respective phases of the latter and scattering the incident particles of the beams in bunches in distinct directions. The beam particles scattered in at least one of these directions are blocked by a diaphragm disposed beyond the foil.

United States Patent inventor Walter Iloppe Schiller-stun: 46, 8000Munich I5, Germany Appl. Nov 813,629 Filed Apr. 4, I969 Patented July27, I97] Priority Apr. 16, I968 Switzerland 5,586/68 PARTICLE BEAMAPPARATUS HAVING AN IMAGING LENS WHICH IS PROVIDED WITH AN ASSOCIATEDI'IIASE DISPLACING FOIL l8 Claims, 2 Drawing Figs.

US. Cl 250/495 A [56] References Cited UNITED STATES PATENTS 3,469,0969/ I 969 Hanssen 250/49. 5 3,500,043 3/1970 Hanssen 250/495 PrimaryExaminerWilliam F. Lindquist Attorneys-Curt M. Avery, Arthur E. Wilfond,Herbert L.

Lerner and Daniel .I. Tick ABSTRACT: A particle beam device has alongitudinal axis and a beam-generating portion for issuing particlebeams along the axis. A holder is provided for accommodating a specimenin the path of the beams and a particle beam imaging lens is disposedbeyond the specimen locality coaxial with the axis. A foil is disposedin the lens in the path of the particle beams for shifting therespective phases of the latter and scatlering the incident particles ofthe beams in bunches in distinct directions. The beam particlesscattered in at least one of these directions are blocked by a diaphragmdisposed beyond the foil.

PATENIEU JUL27I97| 3 596 090 SHEEI 1 BF 2 Fig. 1

PATENTFJ] JUL27 ISH SHEET E OF 2 Fig. 2

PARTICLE BEAM APPARATUS HAVING AN IMAGING LENS WHICH IS PROVIDED WITH ANASSOCIATED PHASE-DISPLACING FOIL My invention relates to particle beamapparatus having a radiation portion normally incorporatingbeam-generating and condenser systems. The apparatus is also equippedwith at least one imaging lens arranged in the beam path beyond the '0specimen, and associated with the lens is a foil for displacing orshifting the phases of the imaging particle beams. The foil can becommonly assigned to several lenses. However, as a rule, it ispreferable to eliminate any disturbance in the particle beam caused byerrors in the first image lens by arranging a foil directly behind thelens. This arrangement is preferred because image lenses in beamdirection magnify the influences caused by errors of the first lens,these errors being especially pronounced relative to the quality of theimage.

In light optics as well as in particle-beam apparatus it is known thatcertain phase conditions occurring during the imaging of objects orspecimens are established by using phase-displacing devices, which forparticle beam apparatus are phase displacing foils. In this connectionreference may be had to: The proceedings of the European RegionalConference on Electron Microscopy," Delft 1960, pages 18 to 24. Thesephase-displacement devices do not create any fundamentul problems whenused in light optics. However, this is not the case with particle beamapparatus of which the principal representatives are the electronmicroscope and diffraction devices.

It is known that the phase-displacing foils produced from the materialscustomarily used for this purpose not only provide the desired phasedisplacement, but, also diffusely scatter the particles such that thescattered particles are superimposed upon the particles which containthe actual information at the image locality thereby reducing thequality of the image.

However, in the production of electron-microscopic recordings orphotographs of objects or specimens at high resolving powers, forexample, it is necessary to ensure that the beams diffracted in thespecimen have precisely defined phase conditions relative to the primarybeam. This is of particular importance in connection with theutilization of the phase-contrast effect during electron-microscopicinvestigations which are conducted on thin specimens carried at highresolution and small radiation aperture. In such specimens, the particlebeam which is used to investigate or form the image is not influencedwith respect to the amplitude of but rather, with respect to the phaseof the impinging waves. This occurs because the specimen details producea potential distribution within the specimen itself.

According to the rule of Zernicke, the beams diffracted in the specimenof at least a positive and negative first order must have a phasedisplacement relative to the primary beam of n'180 if the phase specimenis to provide an amplitude image, the quantity :1 being a whole numberincluding zero.

On the other hand, the diffracted beams in any case have a phasedisplacement of 90 with respect to the primary beam. Therefore, in orderto fulfill the aforementioned condition, the diffracted beams must bephase-displaced by an additional 90, either positively or negatively.With a large aperture for individual lens zones, such phasedisplacements can be produced merely by wave aberrations of the lenswhich are caused by apertural errors and any defocusing which may bepresent.

With reference to the undesired phenomenon already referred to, namely,that relating to the occurrence of omnidirectional scattering ofparticles when using phase-displacement foils in particle beamapparatus, an additional phase displacement is dispersed with andaccording to German Pat. No. l,222,603, a diaphragm having severalelectronperrneable and electron-nonpermeable zones is placed in the pathof the beam passing from the objective lens of an electron microscopeand arranged in such a manner that the diaphragm permits only theelementary waves issuing from the image-side wave surface of the lenssystem which are of unidirectional phase to arrive at an arbitrary pointof incidence. The word unidirectional relates to the phases of thosewaves whose associated space frequencies are imaged or reproduced eitherwith positive or with negative contrasts only. With this diaphragmarrangement, known as a zone diaphragm, the waves which do not meet thephase conditions required for forming an image can therefore not arriveat the image plane.

A certain disadvantage of the zone diaphragm becomes evident when theprinciples of imaging which utilize the phase-contrast effect,discovered during the past few years, are considered. It was foundconvenient in theoretical investigation to treat the specimen accordingto Fourier as being combined of sinusoidal phase lattices of variousspace frequencies. The total of all space frequencies represents thetotal of all specimen points. In the present case, it is of specialimportance that different zones of the image lens plane are responsiblefor transmitting various space frequencies into the image plane. Withrespect the known zone diaphragm, this fact indicates that the maskingof specific waves can sometimes result in certain losses of information.This is recognized in the publication of the Sixth InternationalCongress for Electron Microscopy held in Kyoto, I966, wherein page 39there is suggested an imaging method that uses such zone diaphragms, Inthis connection, a plurality of zone diaphragms of respectivelydifferent dimensions are used at different focusing conditions to makephotographs in a sequence of the same specimen regions. Thesephotographs were superimposed to form a composite in which as many aspossible of the space frequencies contribute to produce the resultingimage.

It is an object of my invention to provide a foil which eliminates theabove disadvantages associated with the known phase-displacing devices.

It is another object of my invention to provide a foil for shifting thephases of imaging particle beams. More specifically, it is an object ofmy invention to provide a foil which scatters the particles impinging onthe specimen in bunches in discrete directions.

To achieve these objects and according to a feature of the invention, Iprovide a foil made of a material which scatters the impingingparticles, in bunches, in distinct directions. In addition, l arrangebeyond the foil in the beam path diaphragms which block the scatteredparticles.

Hitherto, the foil materials used produced diffuse scattering of thebeam particles, whereas the present invention employs a foil with apreferential structure so that although there is inevitably somescatter, this is in discrete spatial directions and is also accompaniedby bunching of the beam particles. In contrast to known foils for thiskind of application, therefore it is now possible to block the scatteredbeam particles, so that only the central beam is effective in generatingthe image, the particular appropriate phase conditions still beingmaintained.

Generally speaking, the correcting foil will be of crystal so that thebunching and scattering of the corpuscles is depen dent upon the crystalstructure. A monocrystal foil has been found to be particularlysuitable, especially since foils of this kind can be produced with aparticularly flat surface.

However, this does not exclude the use of other materials for the foils,for example synthetic materials having a structure such that the scatterangles are adequate for blocking purposes.

If a crystalline foil is used, this may be of graphite or silicon, forexample. The correcting foil can be arranged in the rear focal plane ofthe lens in just the same way as the known zone diaphragm.Alternatively, it can be located in the plane of an aperture diaphragmassociated with the lens, this even if the diaphragm is not located inthe focal plane, so that the provi sion of an additional mounting forthe foil is rendered unnecessary. The aperture diaphragm can beconstructed to contain the foil.

The Zernike condition has already been referred to hereinbefore, inaccordance with which the beams diffracted in the specimen should have aphase of H with respect to the primary beam, and it was also explainedthat fundamentally there is only a phase difference of 90. It ispossible, as those skilled in the art will appreciate, to make thethickness of the correcting foil used in accordance with the inventionconstant, and therefore to achieve a constant phase-shifting effect inall zones of the foil, with the exception of the central zone throughwhich the primary ray passes without undergoing any phase shift. inaccordance with the condition just referred to the foil thickness willbe so chosen that the phase shift of 90 is positive or negative.

This kind of dimensioning of the foil, however, generally onlyapproximately satisfies the phase condition because the beams havealready undergone phase shift as a consequence of the wave aberration ofthe particular lens involved, which phase shift will be dependent uponthe distance from the lens axis. Thus, if the above phase condition isto be satisfied exactly, then the thickness of the foil and thereforethe phaseshifting efi'ect which it produces, must be chosen differentlyin different areas of the foil in order that the phases (determined bythe wave aberration of the lens and the phase-shifting effect of thefoil) of all the particle beams have approximately the same valuerelative to the primary beam. In accordance with what has been said inthe foregoing, this value will be n-lSO where n=0, 1, 2,...etc.

The known type of zone diaphragm may be modified in such a way that thezones thereof which are impermeable or opaque to the beam particles arereplaced by zones which produce a phase-shifting effect relative to thebeams passing through the permeable or transmissive zones, themodification being provided by having the foil of the invention containa pattern corresponding to that of the zone diaphragm. The phase shiftin these zones must, to accord with the laws governing exclusivelypositive or negative contrast in reproduction, be l80 or multiplethereof. Outside these zones, the foil will be so dimensioned that ontransition to the neighboring impermeable zones, no changes of phaseoccur. As is well known, in the conventional form of zone diaphragm,penneable and impermeable annular zones alternate with one anotherprovided that the associated lens has no axial astigmatism; failingthis, there are departures from the circular form, to configurationswhich are roughly elliptical in a first approximation. In order toproduce a zone diaphragm correcting foil first of all a negative in theform ofa metal foil can be produced and then, by using ion etchingthrough said negative, a monocrystalline foil can be etched away atthose areas to which the negative allows the ion beam to pass. Thisprocess can be applied in a corresponding manner to the manufacture of avariable thickness foil.

Up to now, the chief emphasis has been placed upon the design anddimensioning of the phase-shifting foil. The diaphragms provided toblock the scattered beam particles can be located anywhere behind thefoil, considered in the direction of the radiation, provided they areintended simply to block the scattered particles without disturbing themain image, i.e. the image of zero order. In the embodiment to beillustrated presently there is provided, in the plane of the intermediate images produced by the beam particles scattered by the foil, afield-of-view diaphragm which blocks these intermediate images butpasses the main image. Although, when using a polycrystalline foil, thesecondary images are superimposed upon one another in the Deby-Scherrorring, as those skilled in the art will appreciate, this is of nosignificance here because the secondary images are blocked out.

lt is of course essential that the main image and the secondary imagesshould not overlap one another. For this reason, the beam-generatingsection of the apparatus should be arranged to direct onto the specimena particle beam of such a slight cross section (fine zone illumination),that the different images are located in separate areas of theintermediate image plane in which the diaphragm is located.

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 schematically illustrates those pans of the particle beamapparatus which are essential to the invention; and

FIG. 2 is a broken-out view of a microscope column in which isillustrated, in section, an imaging lens provided with an associatedphase-displacing foil according to my invention.

In FIG. 1, an object 1 is irradiated by a fine electron beam. After theobject 1, the beam passes through an objective lens 2, illustrated inpurely schematic fashion, which may be an electrostatic orelectromagnetic lens, and which may be followed by other lenses, notshown. in the image-side focal plane of the lens 2 a phase-shiftingcorrecting foil 3 is located, and in this particular example this is amonocrystalline foil. In the intermediate image plane there is a mainimage 4 produced by the central beam, and also secondary images 5 and 6produced by electrons scattered in the foil 3. Due to the structure ofthe particular material used for the foil 3, the electrons are scatteredin a bunched way in discrete spatial directions. However, the secondaryimages 5 and 6, and any other secondary images which may occur, butwhich have not been shown in the figure, can be blocked and in thisembodi ment this is achieved by a ficld-of-view diaphragm 7 arranged inthe intermediate image plane, which passes the central beam, saiddiaphragm 7 having individual zones which, under the influence of thephase-shifting effect of the foil 3, present the requisite phase valuesfor image production, and being employed to obtain the electronmicroscopic image of the specimen.

PK]. 2 illustrates the relevant part of an exemplary electron microscopeembodiment. The microscope has a column ll in which there is located anobjective lens 12, for the magnified reproduction of an object held invacuum-sealed fashion in an object cartridge 13. The essentialcomponents of the objective lens 12 are an upper pole piece 14 and alower pole piece 15 between which the lens gap is located. In this casean elec tromagnetic lens is utilized having winding 16 which develops aflux that passes through an iron circuit l7, the two pole pieces 14 andI5 and the lens gap.

In the vicinity of the lens gap, a nonmagnetic perforated plate 18 isprovided for the passage of a diaphragm drive system 19 carrying anaperture diaphragm 110. The aperture diaphragm is so constructed that italso functions as the mounting for a phase-shifting foil 111, forexample a monocrystalline foil, which foil may be designed as a zonediaphragm. A drive 112 provides for the transverse displace ment of thediaphragm 110.

After the objective lens 12 in beam direction, there is disposed afurther nonmagnetic ring 113, for example of brass, which locates amounting "4 for a field-of-view diaphragm 115, which in this embodimentblocks the electrons scattered in the foil "1. This mounting 114 isprovided with a drive arrangement "6 which extends in vacuum-tightfashion through the wall ofthe column 11.

The diaphragms serving to block the beam particles scattered in thefoil, can comprise parts of the corpuscular beam apparatus which arealready present, such as suitable flanges or projections.

Upon studying this disclosure it will be obvious to those skilled in theart that my invention is amenable to various modifications with respectto details and can be given embodi ments other than that particularlyillustrated and described herein, without departing from the essentialfeatures of my invention and within the scope of the claims annexedhereto.

lclaim:

t. In a particle beam device which has a longitudinal axis,beam-generating means for issuing a particle beam along said axis, meansfor accommodating a specimen in the path of said beam, an electroopticalimaging lens disposed beyond the specimen locality coaxial with saidaxis, foil means having a crystalline structure and being disposedbeyond the specimen locality in the path of said particle beam forshifting the phases of the resulting diffracted beams and for scatteringthe incident particles of said diffracted beams in bunches in distinctdirections, and blocking means disposed beyond said foil means forblocking the beam particles scattered in at least one of said distinctdirections.

2. The combination of claim 1 wherein said particle beam is an electronbeam.

3. The combination of claim 2 wherein said particle beam device is anelectron microscope.

4. The combination of claim 1 wherein said foil means is amonocrystalline foil.

5. Tile combination of claim 1 wherein said foil means ifa foilconsisting of crystalline graphite.

6 The combination of claim 1 wherein said foil means is a foilconsisting of crystalline silicon.

7. The combination of claim 1 wherein said imaging lens has two focalplanes, one of said focal planes being spaced from said beam-generatingmeans a larger distance than the other of said focal planes, said foilmeans being disposed in said one focal plane.

8. The combination of claim 1 wherein said imaging lens has an aperturediaphragm disposed therein, said foil means being arranged in the planeof said diaphragm.

9. The combination of claim 1 wherein said particle beam issuing fromsaid beam-generating means has beyond the specimen locality a primarycentral portion on said axis and portions diffracted by a specimenplaceable at said specimen locality, and wherein said foil means is afoil of constant thickness so that the phase of said diffracted portionsincident thereon are shifted in phase the same amount except for a cen'tral region in said foil through which said central portion of saidparticle beam passes without being shifted in phase.

10. The combination of claim 9 wherein said thickness of said foil isselected for shifting the phase of said diffracted portions incidentthereon by 90.

11. The combination of claim 1 wherein said particle beam issuing fromsaid beam-generating means has beyond the specimen locality a primarycentral portion on said axis and portions diffracted by a specimenplaceable at said specimen locality, said diffracted portionssurrounding said central portion and having different phases resultingfrom wave aberration in said lens and phase shifting in said foil, andwherein said foil means is a foil having a thickness varying alongdirections radial of said axis, so that said different phases haveapproximately the same value relative to said central portion.

The combination of claim 11 wherein said phase value is substantially nlwhere n is a whole number or zero.

13. The combination of claim l and wherein said scattered beam particlesform a main and secondary images in a plane beyond said foil means, saidblocking means being disposed in said plane for blocking said secondaryimages and passing said main image.

14. The combination of claim 13 wherein said beamgenerating means issuesa fine-zone illumination beam, and said main and said secondary imagesare situated in respective mutually separate regions of said plane.

15. The combination of claim 1 wherein said foil means is a diaphragmhaving mutually separate beamtransmissive and beam'opaque zones.

16. The combination of claim 1 wherein said foil means is a diaphragmhaving mutually separate beam-transmissive and beam-phaseshifting zones.

17. The combination of claim 16 wherein at least one of saidbeam-phase-shifting zones has a thickness corresponding to a shift ofthe phase of a beam incident thereon by 18. The combination of claim 1wherein said blocking means includes an auxiliary diaphragm for blockingsaid beam particles scattered in at least one of said distinctdirections.

1. In a particle beam device which has a longitudinal axis,beam-generating means for issuing a particle beam along said axis, meansfor accommodating a specimen in the path of said beam, anelectro-optical imaging lens disposed beyond the specimen localitycoaxial with said axis, foil means having a crystalline structure andbeing disposed beyond the specimen locality in the path of said particlebeam for shifting the phases of the resulting diffracted beams and forscattering the incident particles of said diffracted beams in bunches indistinct directions, and blocking means disposed beyond said foil meansfor blocking the beam particles scattered in at least one of saiddistinct directions.
 2. The combination of claim 1 wherein said particlebeam is an electron beam.
 3. The combination of claim 2 wherein saidparticle beam device is an electron microscope.
 4. The combination ofclaim 1 wherein said foil means is a monocrystalline foil.
 5. THecombination of claim 1 wherein said foil means if a foil consisting ofcrystalline graphite.
 6. The combination of claim 1 wherein said foilmeans is a foil consisting of crystalline silicon.
 7. The combination ofclaim 1 wherein said imaging lens has two focal planes, one of saidfocal planes being spaced from said beam-generating means a largerdistance than the other of said focal planes, said foil means beingdisposed in said one focal plane.
 8. The combination of claim 1 whereinsaid imaging lens has an aperture diaphragm disposed therein, said foilmeans being arranged in the plane of said diaphragm.
 9. The combinationof claim 1 wherein said particle beam issuing from said beam-generatingmeans has beyond the specimen locality a primary central portion on saidaxis and portions diffracted by a specimen placeable at said specimenlocality, and wherein said foil means is a foil of constant thickness sothat the phase of said diffracted portions incident thereon are shiftedin phase the same amount except for a central region in said foilthrough which said central portion of said particle beam passes withoutbeing shifted in phase.
 10. The combination of claim 9 wherein saidthickness of said foil is selected for shifting the phase of saiddiffracted portions incident thereon by 90*.
 11. The combination ofclaim 1 wherein said particle beam issuing from said beam-generatingmeans has beyond the specimen locality a primary central portion on saidaxis and portions diffracted by a specimen placEable at said specimenlocality, said diffracted portions surrounding said central portion andhaving different phases resulting from wave aberration in said lens andphase shifting in said foil, and wherein said foil means is a foilhaving a thickness varying along directions radial of said axis, so thatsaid different phases have approximately the same value relative to saidcentral portion. The combination of claim 11 wherein said phase value issubstantially n.180*, where n is a whole number or zero.
 13. Thecombination of claim 1 and wherein said scattered beam particles form amain and secondary images in a plane beyond said foil means, saidblocking means being disposed in said plane for blocking said secondaryimages and passing said main image.
 14. The combination of claim 13wherein said beam-generating means issues a fine-zone illumination beam,and said main and said secondary images are situated in respectivemutually separate regions of said plane.
 15. The combination of claim 1wherein said foil means is a diaphragm having mutually separatebeam-transmissive and beam-opaque zones.
 16. The combination of claim 1wherein said foil means is a diaphragm having mutually separatebeam-transmissive and beam-phase-shifting zones.
 17. The combination ofclaim 16 wherein at least one of said beam-phase-shifting zones has athickness corresponding to a shift of the phase of a beam incidentthereon by 180*.
 18. The combination of claim 1 wherein said blockingmeans includes an auxiliary diaphragm for blocking said beam particlesscattered in at least one of said distinct directions.